Helicobacter pylori is a Gram-negative bacterium that colonizes the human stomach. H. pylori infection is associated with an increased risk of cancer of the distal stomach, as well as peptic ulcer disease. The World Health Organization has classified H. pylori as a type I carcinogen, and gastric cancer is the third leading cause of cancer-related death worldwide. The long-term goals of this work are to understand the molecular mechanisms that allow H. pylori to persistently colonize the human gastric mucosa, to understand the molecular mechanisms by which H. pylori infection leads to the development of gastric cancer or peptic ulceration, and to develop effective strategies for the prevention of these diseases. To achieve these long-term goals, we seek to understand the actions of bacterial proteins that are localized on the surface of H. pylori. H. pylori genomes contain more than 50 genes that are predicted to encode outer membrane proteins (OMPs). Several OMPs have been reported to mediate H. pylori adherence to gastric epithelial cells, but the functions of most H. pylori OMPs are not known. The overall hypothesis of this proposal is that H. pylori utilizes specific OMPs at various stages of the infectious process to optimize initial colonization of the stomach and to facilitate persistent colonization in the presence of a gastric mucosal inflammatory response, thereby contributing to the development of gastric disease. The specific aims are (i) To define the role of two- component signal transduction systems (TCSs) in regulating genes encoding OMPs, (ii) To define OMPs that have a dominant role in promoting H. pylori colonization of the stomach, and (iii) To define temporal features of processes by which specific OMPs promote H. pylori colonization of the stomach and modulate development of gastric disease. To accomplish Aim 1, we will compare the transcriptomes of wild-type and mutant strains, using RNA-seq and quantitative RT-PCR methods. To accomplish Aim 2, we will infect mice with a library of strains containing mutations in OMP-encoding proteins, each labeled with a distinct nucleotide bar code, and then will use high throughput sequencing to analyze the bacterial populations colonizing the stomach. To accomplish Aim 3, we will regulate the expression of selected OMP-encoding genes in vivo through use of an inducible promoter. Collectively, these experiments will provide important new insights into the roles of specific OMPs in promoting initial colonization of the stomach, persistence and disease.